Structural Changes and Spectroscopic Properties of Ce3+-Ion-Doped

Dec 17, 2009 - Energy transfer study between Ce3+ and Tb3+ ions in doped and core-shell sodium yttrium fluoride nanocrystals. Pushpal Ghosh , Arik Kar...
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J. Phys. Chem. C 2010, 114, 715–722

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Structural Changes and Spectroscopic Properties of Ce3+-Ion-Doped Sodium Yttrium Fluoride Nanocrystals: Influences of Sonication and Temperature Pushpal Ghosh, Arik Kar, and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: September 16, 2009; ReVised Manuscript ReceiVed: December 3, 2009

The formation and decomposition of a layered structure of ammonium fluorolanthate during the preparation of Ce3+-doped sodium yttrium fluoride nanomaterials are investigated by XRD, TGA, TEM, FTIR, and photoluminescence spectroscopy. The influences of sonication and temperature of heating on the structural changes, that is, lattice strain and lattice parameters due to decomposition of the layered fluoride structure, have been demonstrated. The change of morphology from nanorods to nanoparticles after sonication is confirmed by TEM study. It is confirmed from FTIR study that interlayer water molecules are “icelike connective water” in nature and it generates strong crystal field effects on Ce3+ ions. The Ce3+ ion is used as a spectroscopic probe to confirm the presence of interlayer water and its influences on spectroscopic properties of Ce3+ ions. Introduction Layered structure materials have drawn considerable attention due to their various potential applications, such as superconductivity,1 photocatalytic activities of interlayer nanospace for decomposition of water,2 or intercalation and deintercalation ability utilized in battery applications.3 It has been reported that Ru(bpy)32+ in the interlayer yielded a significant amount of photocurrent in Ti and Nb layered oxides.4 Ozawa et al. reported the photoluminescence properties of the Eu3+-activated double perovskite type layered phosphor (K1.5Eu0.5)Ta3O10 and Eu0.56Ta2O7 nanosheets with high photoactivator concentration.5 A significant enhancement of the photoluminescence (PL) of Eu3+ ions in the interlayers of Ti layered oxide is due to interlayered water molecules.6 The blue photoluminescence of an oxide mononanosheet derived from layered perovskite Bi2Sr2Ta2O9 (BST) was reported by Matsumoto et al.6,7 Layered materials are recently extensively used in photocatalysis; for example, Sn2+-ion-exchanged layered titanates and niobates are helpful for photocatalytic H2 and O2 evolution reactions.8 Mallouk and co-workers reported layered single-crystal spinel NiCoAlO4 and NiCo2O4 platelets that exhibit room-temperature superparamagnetism.9 Linear and layered organically templated iron sulphates and inorganic fullerene-like structures of layered Hf2S were reported by Rao et al.10 Liu et al. reported novel nanocage structures derived from carboxy methyl β-cyclodextrins (CMCDs) intercalated in layered double hydroxides (LDHs).11 Transformation of layered zinc phosphate to a threedimensional open framework structure has been recently explored.12 Organic-inorganic hybrid layered manganese oxide nanocomposites are also important precursors for mesoporous manganese oxides for catalysis applications.13 Ammonium fluorolanthanates are another group of interesting materials because of their crystal structure. Rajeshwar et al. reported two main types of fluorolanthates, (NH4)3Y2F9 and NH4Y2F7, and the structure of NH4Ln2F7 can be visualized as layers of LnF6 octahedron (built up by mutual sharing of corners and edges) with interleaving layers of NH4 groups.14 Huang et * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (91)-33-2473-4971. Fax: (91)-33-2473-2805.

al.15 discussed the layered structure of NH4LnF4 (Ln) (Nd, Sm, Eu, Gd, and Tb), which is potentially stacked along the (100) direction linked by NH4 + ions. Meyer and Plitzko16 synthesized and determined the crystal structure of layered NH4DyF4 and NH4MF4. Again, Liang et al. reported ternary NH4Ln2F7 inorganic nanocages and ascertained that these new type nanostructures are believed to have a close relationship with their inherent layered structure, similar to that of inorganic fullerene-like nanoparticles.17 Recently, a great attention has been focused on rare-earthdoped sodium yttrium fluoride nanocrystals for efficient luminescent materials.18,19 The NaYF4 exists in two polymorphs at ambient temperature and pressure, the hexagonal structure and the cubic one, depending on the synthesis conditions.20 Kra¨mer et al.21 found the gagarinite structure for a slightly nonstoichiometric mixed Na3-xLn2xF6 (x ) 0.45). According to Burns,22 there are three types of cationic sites in a hexagonal sodium yttrium fluoride: a one-fold site occupied by Y3+, a one-fold site occupied randomly by 1/2 Na+ and 1/2 Y3+, and a twofold site occupied randomly by Na+ and vacancies of the structure of hexagonal NaYF4, and the fluoride coordination about the first two sites is nine-fold. The main motivation of this work is to study the formation and decomposition of layered structure fluoride compounds using sonication and heating temperature. In the present study, rare-earth-doped hexagonal sodium yttrium fluoride with a small amount of ammonium fluorolanthates [(NH4)3Y2F9 and NH4Y2F7] by a reverse micelle technique have been prepared. XRD, TEM, and FTIR studies have been done to understand the decomposition of the layered structure that is formed between sodium yttrium fluoride and ammonium fluorolanthates. To the best of our knowledge, there is no report on the influences of sonication and temperature of heating on the structural changes, that is, lattice strain and lattice parameters due to decomposition of layered fluoride structure. The interlayer water molecules are confirmed by spectroscopic studies. These water molecules may generate strong crystal field effects on Ce3+ ions. The Ce3+ ion is being used as a spectroscopic probe to confirm the presence of interlayer water.

10.1021/jp910912c  2010 American Chemical Society Published on Web 12/17/2009

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Experimental Section Preparation of Na(Y1.5Na0.5)F6: Nanorods/Nanoparticles. A hydrothermal method for the synthesis of sodium yttrium fluoride nanocrystals using nonionic surfactant Span 80 (sorbitan monooleate) has been demonstrated earlier.20a In the present study, a simple microemulsion method using cationic surfactant, CTAB (cetyl trimethyl ammonium bromide), has been used. Two sets of microemulsions (A and B) were prepared through water-in-oil (w/o) type emulsion with cetyl trimethyl ammonium bromide (CTAB) (ALFA AESAR) and isooctane as the cationic surfactant and organic liquid phase, respectively. CTAB (0.456 M, 9.475 g), 38 mL of isooctane (Merck), 10 of mL 1-butanol, and 3.5 mL of water are mixed together to prepare the microemulsion A. Similarly, the B set is prepared by the same composition. Here, 1-butanol is used as cosurfactant. In 3.5 mL of water, Y(NO3)3 · 6H2O, (0.425 M, Indian Rare Earth Ltd.), 0.425 M NaCl (Merck), and a required amount of cerium nitrate (for 1.0 mol % Ce2O3, Aldrich) were mixed and stirred for a few minutes. The solution mixture was then added to A and stirred for 30 min. The amount of water is adjusted to keep the water-to-surfactant ratio (w/o) at 15. NH4F solution (1.7 M) was added to the other set of microemulsion B and stirred for 30 min. Now, the fluoride-containing microemulsion is added to the previous emulsion and stirred for 1 h. In this preparation technique, the Y3+/F- ratio and pH of the solution were maintained at 1:8 and 6-7, respectively. Finally, a white precipitate was obtained. Particles were then collected by centrifugation (6000 rpm); then the particles were washed twice with acetone and methanol and dried at 75 °C for 12 h in a vacuum oven, and the samples were collected. Finally, these samples were heated at 200, 300, 400, and 600 °C for 1 min with a heating rate of 12 °C/min. Sonication was carried out in an open beaker that was kept in an ice bath. The sonication times are 5, 10, 20, and 40 min. The water was used as solvent. After completing sonication, the particles were washed and dried at 75 °C for 12 h in a vacuum oven. Transmission electron microscopy (TEM) (JEOL model 200) was used to characterize the samples. The crystallization of the samples was characterized by X-ray diffraction (XRD) using a Siefert XRD 3000 P. The crystallite sizes of the nanocrystals were calculated following the Scherrer equation

D ) Kλ/β cos θ

(1)

where K ) 0.9, D represents crystallite size (Å), λ is the wavelength of Cu kR radiation, and β is the corrected half width of the diffraction peak. The excitation and emission spectra were recorded in a Fluoro MaX-P (Horiba Jobin Yvon) spectrometer, using a solid sample holder at room temperature. Thermogravimetric and differential thermal analysis (TG and DTA) were obtained from SDT (Quanta chrome) with the heating rate of 12 °C/min using N2 gas passing. Infrared spectra of the samples (FTIR) were recorded in the range of 400-4000 cm-1 on a Fourier transformation spectrometer (Nicolet Magna IR 750 series 2). A small amount of sample was mixed with KBr and pressed to make a thin pellet for FTIR studies. Samples were sonicated at 37 kHz with a frequency sonicator made by Toshniwal Process Instruments Private Limited. Results and Discussion Figure 1 depicts the crystal structure of as-prepared (75 °C) Ce3+-doped Na(Y1.5Na0.5)F6 nanocrystals with varying the sonication time. Figure 1, pattern a, shows the XRD pattern of

Figure 1. X-ray powder diffraction patterns of 75 °C dried 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals: (a) as-prepared, (b) sonicated for 5 min, (c) sonicated for 10 min, and (d) sonicated for 20 min. (The star marked peaks are due to formation of (NH4)3Y2F9.)

Figure 2. X-ray powder diffraction patterns of the (201) peak of hexagonal 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals: (a) as-prepared, (b) sonicated for 5 min, (c) sonicated for 10 min, and (d) sonicated for 20 min.

1.0 mol % Ce3+-doped as-prepared Na(Y1.5Na0.5)F6 nanocrystals. The peaks at 17.03° (100), 29.76° (110), 30.54° (101), 34.55° (200), 39.36° (111), and 43.22° (201), (JCPDS card no.16-334) clearly suggest the formation of a Na(Y1.5Na0.5)F6 hexagonal phase. The peaks marked with the asterisks, such as 12.14° (100), 26.12° (-201) and 26.96° (120), suggest the formation of monoclinic ammonium fluorolanthate (NH4)3Y2F9 (JCPDS card no. 43-0840). The peak at 12.14° (asterisk marked) is the 100 intensity peak of (NH4)3Y2F9. It reveals that both Na(Y1.5Na0.5)F6 (major) and (NH4)3Y2F9 (minor) are present at 75 °C dried samples. The calculated lattice volumes are 118.394 and 651.76 Å3 for Na(Y1.5Na0.5)F6 and (NH4)3Y2F9, respectively. The lattice volume of Na(Y1.5Na0.5)F6 is ∼5.5 times less than that of (NH4)3Y2F9. Figure 1, patterns b-d, depicts the XRD patterns of 1 mol % Ce3+-doped as-prepared Na(Y1.5Na0.5)F6 nanocrystals sonicated for 5, 10, and 20 min, respectively. With increasing the sonication time, the marked peaks designated for (NH4)3Y2F9 decrease with time and finally disappear after 10 min of sonication. After careful analysis, it is seen that the peaks for Na(Y1.5Na0.5)F6 are shifted to the higher angle with increasing the sonication time. Here, we have considered that only the highest intensity peak corresponds to the (201) plane of Na(Y1.5Na0.5)F6 for peak-shifting analysis (Figure 2). Liu et al.11 confirmed the inclusion of dodecyl benzene in cyclodextrinintercalated layered double hydroxide on the basis of XRD peak shifting. It is reported that the (001) peak position of the

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TABLE 1: Preparation Conditions, Crystallite Size, and Cell Parameters of 1.0 mol % Ce3+-Doped Sodium Yttrium Fluoride Nanocrystals sample name Na(Y1.5Na0.5)F6:Ce(1)

temperature (°C) (sonication time)

crystallite size (nm)

75 (nil) 75 (5 min) 75 (10 min) 75 (20 min) 75 (40 min) 200 (nil) 300 (nil) 400 (nil) 600 (nil)

21.1 23.3 24.2 26.7 28.3 21.3 21.3 21.6 56

H1-xNb(1-x)Mo1+xO6 sample was shifted to lower angles with increasing niobium content due to increasing the interlayer space.23 The peak shifting at higher angle of iron oxide-layered titanate nanohybrids with increasing temperature indicates the collapse of the intercalation structure.24 The diffraction lines of NH4AlF4 are shifted to higher 2θ values during preparation of γ-AlF3 from layered NH4AlF4.25 Similar peak shifting in layered structures is also reported by Hosogi et al.8 and Gunawan et al.26 In the present study, it is clearly evident from Figure 2 that the diffraction lines for Na(Y1.5Na0.5)F6 are shifted gradually to the higher angles with increasing sonication time. Another way to prove the layered structure is to study the lattice parameter or cell parameter. Ozawa et al.5 showed that the large lattice parameter “b” of the as-prepared sample is considered to indicate the existence of the interlayer water for the system (K1.5Eu0.5)Ta3O10 double perovskite structure, and the lattice parameter “c” decreases with increase in temperature. Alonso et al.25 showed that parameter c decreases with increasing the temperature with the loss of water for the system (NH4)3AlF6, NH4AlF4. In the present study, it is seen that the c value for Na(Y1.5Na0.5)F6 decreases with increasing the sonication time, suggesting the decomposition of the layered structure (Table 1). Again, the c value increases after 10 min of sonication, that is, after destroying the layered structure. Therefore, we can assume that the fluoride ion of most of the Na(Y1.5Na0.5)F6 nanocrystal is connected to the NH4+ ion of (NH4)3Y2F9 and forms a weak N-HfF bond along the c axis. This layered structure decomposes gradually on sonication. Analysis results show that there is a possibility to form layered structures between major sodium yttrium fluoride and minor ammonium fluorolanthate that decomposes on sonication. Temperature is one of the important parameters that influences tuning of the crystal structures of nanocrystals. Figure 3 shows the XRD patterns of 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals heated at different temperatures. Figure 3, pattern a, shows the XRD pattern of 75 °C dried samples and both hexagonal Na(Y1.5Na0.5)F6 (major) and monoclinic (NH4)3Y2F9 (minor) are present. As the temperature is increased to 200 °C, (Figure 3, pattern b), the peaks due to hexagonal crystal phase remain the same. The peaks marked with the positive sign, such as 13.11° (200), 14.98° (210), 25.93° (021), 27.355° (410), and 32.646° (422), indicate the formation of cubic NH4Y2F7 (JCPDS card no. 43-0847). The peak at 27.41° (+ marked) is known to be the 100 intensity peak of cubic NH4Y2F7. It indicates that both Na(Y1.5Na0.5)F6 (major) and NH4Y2F7 (minor) are present at a 200 °C heated temperature. The calculated lattice volume for NH4Y2F7 is 2406 Å3, which is much higher than that of Na(Y1.5Na0.5)F6. The layered structure between Na(Y1.5Na0.5)F6 and NH4Y2F7 was proposed previously.20a At the 300 °C heated sample, the intensity of the peaks due to NH4Y2F7 are reduced in intensity. The peaks due

cell parameter (Å) a a a a a a a a a

) ) ) ) ) ) ) ) )

6.189 6.165 5.983 6.153 6.206 6.182 6.182 6.177 6.205

c c c c c c c c c

) ) ) ) ) ) ) ) )

3.569 3.561 3.529 3.619 3.609 3.567 3.562 3.562 3.608

cell volume (Å3)

lattice strain

118.39 117.2 110.1 118.6 120.37 118.07 117.89 117.74 120.31

-0.38% (compressive) -0.21% -0.05% +0.38% tensile +0.42% tensile -0.54% -0.80% -0.66% -0.17%

to NH4Y2F7 phase are eliminated completely at 400 °C. The 100 intensity peak of NH4Y2F7 at 27.41 is shifted to 28.01° with increasing the temperature from 75 to 400 °C. The peak at 28.01° (111) is the 100 intensity peak of orthorhombic YF3 (JCPDS card no. 32-1431; SG, Pnma). A similar result was obtained by Huang et al.15a during the preparation of the 1-D nanostructure LnF3 from NH4LnF4. Rageswar and Seco also reported the formation of DyF3 after heating (NH4)3Dy2F9 at 230 °C.14 Liang et al.17 reported that NH4Ln2F7 (Ln ) Y, Ho, Er, Tm, Yb, Lu) nanocages decompose to LnF3, NH3(g), and HF(g) on heating. Decomposition of ammonium fluoaluminate (structural similarity with ammonium fluorolanthate) releases NH3(g) and HF(g), which was confirmed by thermogravimetric/ mass spectrometry (TG/MS) study.25 Decomposition mechanisms in the present study are given below: 200 °C

(NH4)3Y2F9 98 NH4Y2F7 + 2NH3(g) + 2HF(g)

400 °C

NH4Y2F7 98 2YF3 + NH3(g) + HF(g)

(2)

(3)

Therefore, a two-step decomposition of ammonium fluorolanthate is observed during heating and a single-step decomposition is observed during sonication. Figure 3, pattern e, depicts the XRD pattern of the 600 °C heated sample. Here, all the peaks are well-indexed with hexagonal Na(Y1.5Na0.5)F6 except for the peak at 28.48° (111) for cubic phase. The solid solution

Figure 3. X-ray powder diffraction patterns of 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals heated at different temperatures: (a) 75 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, and (e) 600 °C. (The star (*) marked peaks and positive sign (+) peaks are due to (NH4)3Y2F9 and NH4Y2F7, respectively.)

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Figure 4. Plot of β cos θ/λ against sin θ/λ for 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals for as-prepared (a) and sonicated for 40 min (b).

of YF3 in NaYF4 occurs by substituting Y3+ for Na+ ions and by filling some of the vacancies with F- ions.27 According to Thoma et al.,28 the first eutectic temperature of the NaF.YF3 system is at 638 °C. In the present study, the samples were prepared at 600 °C, which is below the eutectic temperature. The relative intensity of the hexagonal (100) plane compared to the other strong hexagonal peaks increases with increasing temperature. This hexagonal (100) plane becomes most intense compared with the other peaks for the 600 °C heated sample, suggesting the anisotropic growth along the (100) plane. It is clearly seen from Table 1 that the c value decreases with increasing the temperature from 75 to 400 °C, suggesting the layered structure potentially stacks along the direction of the (100) plane linked by a NH4+ ion along the c axis. Again, a slight increase in the c value observed for 600 °C suggests that the layered structure is destroyed entirely after 300 °C. It is seen from Table 1 that the crystallite size remains the same up to 300 °C and it increases at the 600 °C heated sample, indicating that the N-HfF bond breaks with increasing the heating temperature from 75 to 300 °C. Generally, the broadening of the diffraction peaks depends upon strain and particle size. We calculate the strain using Williamson and Hall theorem29

β cos θ/λ ) 1/D + η sin θ/λ

(4)

where β is the full width at half-maximum (fwhm), θ is the diffraction angle, λ is the X-ray wavelength, D is the effective particle size, and η is the effective strain. The strain is calculated from the slope, and the crystallite size (D) is calculated from the intercept of a plot of β cos θ/λ against sin θ/λ. Figure 4a shows the plot of β cos θ/λ against sin θ/λ for 75 °C dried 1 mol % Ce3+-doped Na(Y1.5Na0.5)F6 nanocrystals, considering only the hexagonal peaks. The slope value is -0.0038, which indicates the presence of compressive strain (-0.38%). The calculated crystallite size is 22.2 nm, which matches with the value obtained from the Scherrer equation. The compressive strain values are -0.21% and -0.05% for 5 and 10 min of sonication, respectively (Table 1). This decrease of strain value is due to removal of ammonium fluorolanthate. It is interesting to note that the tensile strain is obtained after 20 and 40 min of sonication (Table 1 and Figure 4b). In the present case, a strong vibrational energy due to sonication is being used to generate the tensile strain. Thus, the lattice strain varies under the influence of sonication. Lattice strain can also be changed with

Figure 5. Plot of lattice strain against temperature (°C) of heating for 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals.

Figure 6. TGA-DTA curve of 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals.

changing the temperature, which is shown in Figure 5. It is seen that the compressive strain increases with increasing the temperature and it reaches a maximum at 300 °C, then further decreases. It is already seen from XRD study that both Na(Y1.5Na0.5)F6 (major) and (NH4)3Y2F9 (minor) coexist at the 75 °C dried samples and (NH4)3Y2F9 (lattice volume ) 651.76 Å3) decomposes to NH4Y2F7 (lattice volume ) 2406 Å3) at above 200 °C. The change of lattice volume may cause the change of lattice strain. It reveals that the sonication has a pronounced effect on crystal structure and lattice strains of nanocrystals. The TGA study will give more insight into the weight loss of water due to change of temperature. Figure 6 shows the TGA and DTA curves of 1.0 mol % Ce3+-doped hexagonal sodium yttrium fluoride nanocrystals. Two weight loss steps can be observed from the TGA curves. The first step (from room temperature to 200 °C), in which there is a 6% weight loss, is due to desorption of water. The weight loss in the temperature range between room temperature and 100 °C is due to removal of relatively free water. The loss in the range between 100 to 250 °C is due to the liberation of bound water or interlayer water.20a Matsumoto et al.6 also showed the loss of intercalated water in a titanate layered oxide structure in the range of

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Figure 7. Low-magnification TEM (a), HRTEM (b), and FFT (c) pattern of as-prepared 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals. Low-magnification TEM (d) and HRTEM (e) of 10 min sonicated and low-magnification TEM (f) and HRTEM (g) of 40 min sonicated 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals. Low-magnification TEM of 300 °C (h) and 600 °C (i) heated sodium yttrium fluoride nanocrystals.

100-400 °C. A significant weight loss in the range of 250-400 °C is due to removal of NH3(g) and HF(g) by the decomposition of NH4Y2F7, which is in good correlation with Liang’s result.17 It is seen that the main weight loss of CTAB surfactant appears after 225 °C (not shown in the text), which matches with earlier literature.30 The samples were washed extensively to remove the surfactant CTAB. Analysis suggests that the weight loss in the range of 100-250 °C is definitely due to bound water or interlayer water, which is formed during the formation of a layered structure of sodium yttrium fluoride. The change in morphology during sonication is studied by TEM. Figure 7a shows the low-magnification TEM image of 75 °C dried 1.0 mol % Ce3+-doped Na(Y1.5Na0.5)F6 nanocrystals layered with ammonium fluolanthate. Hexagonal sodium yttrium fluoride nanorods/nanowires with a high aspect ratio (20) were obtained using CTAB-mediated reverse micelle. Wei et al.31 demonstrated the role of water-to-surfactant ratio (W0) on the morphology of the product. They reported the formation of

nanowires/nanorods at W0 > 10 and spherical particles at W0 < 10 values. In the present study, the nanorods are obtained because W0 ) 15, and this observation agrees with previous results. Figure 7b shows the high-resolution image (HRTEM) of nanorods. The measured interplanar distance supports the formation of the hexagonal phase. The fast Fourier transformation pattern (FFT) in Figure 7c also confirms the presence of hexagonal (211), (101), and (100) planes that match with the XRD result. The low-magnification TEM and HRTEM images (Figure 7d,e) clearly show the formation of the hollow spherical structures after 10 min of sonication, indicating the evaporation of materials due to decomposition of the interlayer. It is clearly seen from TEM images that the average width of the void space is 3-4 nm. The sphere is the most thermodynamically favored shape with the lowest surface energy; therefore, nanorods have a general tendency to be transformed into nanoparticles that have minimum free energy. The transition of Cu(OH)2 nanowires to 3D CuO microstructures under ultrasonic irradiation is

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Figure 8. FTIR spectra of as-prepared (a), sonicated for 40 min (b), 200 °C heated (c), and 400 °C heated (d) 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals.

reported recently.32 The transformation from rod to particle for the NbC and TaC compounds is explained by Rayleigh instability.33 El-Syed and co-workers reported laser-induced shape changes of gold nanorods to nanoparticles using femtosecond and nanosecond laser pulses.34 Morphology changes from rod to sphere by a cation exchange method was reported by Alivisatos and co-workers.35a They have given two possible explanations: One is diffusion kinetics with appropriate chemical and mechanical driving force fields near the heterogeneous interfaces where the reaction occurs. The other one is the propagation of the reaction zone at the interface. In the present study, the change of morphology may be due to diffusion kinetic control because of a high diffusion coefficient of the NH4+ ion. Figure 7f,g depicts the low-magnification and HRTEM images of a 40 min sonicated sample. The void space is much less compared with the 10 min sonicated sample (Figure 7d,e). However, coalescence among particles occurred during the long sonication time (40 min), resulting in less void space. After the HRTEM images are analyzed (Figure 7g), the twinned structures are obtained in a different region, suggesting the “oriented attachment” among the multiple particles occurs via the lowenergy plane (110). Previously, we have reported oriented attachment for the morphological change of LaPO4 nanorods to nanoparticles.35b According to Banfield et al.,35c the oriented attachment involves spontaneous self-organization, followed by joining of the nanoparticles/nanorods at a planar interface. Figure 7h,i depicts the low-magnification images of 300 and 600 °C heated samples, respectively. It is seen that the morphology changes from rods to particles on heating also. The observed void space at 300 °C heated sample of the layered structure is being destroyed upon temperature of heating The FTIR study will help in understanding the nature of OH stretching vibrations for different species. Figure 8, spectrum a, shows the FTIR spectrum of 75 °C dried as-prepared sodium yttrium fluoride nanocrystals. Three strong peaks located at 1400, 1445, and 1473 cm-1 correspond to the splitting of the triply degenerate υ4 of the NH4+ ion.14 This NH4+ ion obviously comes from (NH4)3Y2F9. It is interesting to note that these bands disappeared after sonication (40 min), which further confirmed the decomposition of (NH4)3Y2F9 (Figure 8, spectrum b). A marked difference in -OH stretching vibration (3000-3700 cm-1) is also observed after sonication. The -OH stretching vibration bands at 3104 and 3186 cm-1 are shifted to 3432 cm-1, and the band becomes broadened after sonication. Guo et al.36 reported that the -OH stretching vibration of confined water in a glycolipid nanotube wall is observed at low energy (3300

Ghosh et al.

Figure 9. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of 1.0 mol % Ce3+-doped sodium yttrium fluoride nanocrystals (a) as-prepared and (b) after 40 min of sonication (λex ) 256 nm for as-prepared and 261 nm for sonicated).

cm-1) and the bandwidth becomes narrower comparing with the stretching band of bulk water. Graener and Laubereau,37 demonstrated three kinds of -OH stretching bands, depending upon different environments. The OH stretching band at 3300 cm-1 is attributed to the ice structure that involves at least four strong H bonds with small bond angles (tetrahedral local geometry); the band at 3400 cm-1 is due to the partial formation of H bridges that give a “bifurcated” H bond. The band at 3500 cm-1 is due to weaker bifurcated H bonds. These three contributions of OH stretching vibration can be termed as “connective water”, “intermediate water”, and “multimeric water”, respectively.38 In Figure 8, spectrum a, the position of the -OH stretching vibration suggests that water is “connective” in nature; that means that strong icelike hydrogen bonded water is present. The -OH stretching frequency changes to bulk waterlike after sonication. After heating, a characteristic change is observed for both the peaks of the NH4+ ion and the -OH stretching vibration (Figure 8, spectrum c). The strong peak located at 1432 cm-1 corresponds to the NH4+ ion of NH4Y2F7, which indicates the formation of NH4Y2F7 along with sodium yttrium fluoride at this temperature. Unlike (NH4)3Y2F9 (Figure 8, spectrum a), here, triply degenerate υ4 does not split and matches with previous results.14,20a The -OH stretching vibration for the 200 °C heated sample has two strong bands centered at 3231 and 3439 cm-1. Here, the intensity of these two peaks is almost the same, which suggests that “connective water” and “intermediate water” are present. Crupi et al.39 reported that icelike arrangements tends to decrease when the temperature increases from 5 to 80 °C. After heating at 300 °C, the bands due to -OH stretching and the NH4+ ions are decreased, indicating the destruction of the layered structure due to the decomposition of NH4Y2F7 (figure not shown here). At 400 °C (Figure 8, spectrum d), the peaks due to NH4+ almost vanished, indicating that the layered structure is totally destroyed, and -OH stretching (3418 cm-1) vibration indicates the presence of bulk water in the sample. The above analysis clearly proves the formation and destruction of the layered structure with interlayer/confined water under the influence of sonication and heating temperature. Figure 9 depicts photoluminescence excitation and photoluminescence spectra of the 75 °C dried as-prepared and sonicated Na(Y1.5Na0.5)F6 sample. Two prominent excitation peaks near 256 and 289 nm are observed for the as-prepared sample. The spin-orbit interaction splits the 2F ground state into two J states separated by 2200 cm-1 because the 4f electrons are shielded

Ce3+-Doped Sodium Yttrium Fluoride Nanocrystals

Figure 10. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of (a) as-prepared (λex ) 256 nm), (b) 200 °C heated (λex ) 261 nm), (c) 300 °C heated (λex ) 261 nm), and (d) 600 °C heated (λex ) 261 nm) 1 mol % Ce3+-doped sodium yttrium fluoride nanocrystals.

from the ligand field by the closed 5s and 5p electron shells; therefore, the overall splitting of the 2FJ state is small.38 It is known that the 4f configuration of the Ce3+ ion has only one electron and the irradiation of UV photons will excite this 4f electron in a 5d orbital, leaving the 4f shell empty. Therefore, the excitation spectrum of Ce3+ showed the direct splitting information of the 5d orbital in the crystal field.40 It is seen from Figure 9 that the intensity of the peak at 289 nm of the as-prepared sample decreases with increasing the sonication time and finally disappears after 40 min of sonication. Patra et al.41 reported that the generation of absorption bands of the Ce3+ ion is due to the change of local symmetry and coordinated water molecules to the activator Ce3+ ion. It is also reported the four-fold splitting of Ce3+ after 60 days of aging under humid condition. The degeneracy of the 5d state is partially or completely removed, and the overall splitting of the 5d manifold is typically of the order of 5000-10 000 cm-1, depending upon the site symmetry. Here, the overall splitting of the 5d manifold is 4400 cm-1. It is already established that the interlayer water for the as-prepared samples is “connective” in nature, that is, strong icelike hydrogen bonded water that generates a stronger crystal field. Therefore, the crystal field strength applied on the Ce3+ ion is strongest for as-prepared samples and it lost the degeneracy of the 5d orbital and gave two excitation peaks. When this interlayered water is removed by sonication, the crystal field strength on the activator Ce3+ ion decreases, which is evidenced by the peak shifting to the lower-energy side. In the present study, excitation to the lower-energy level is much more influenced than the higher-energy level. This is because the lower-energy level arises due to splitting of the 5d orbital, which is caused by interlayer or connective water. As this icelike water is gradually removed, the crystal field strength decreases, the degeneracy of the 5d orbital is regained, and we got one main peak located at the higher-energy side for the sonicated sample. As-prepared and sonicated samples are excited by 256 and 261 nm, respectively, and the emission spectrum of Na(Y1.5Na0.5)F6:Ce(1) nanocrystals includes a broad band ranging from 350 to 500 nm, which is assigned to the parity allowed transitions of the lowest component of the 2D state to the spinorbit components of the ground, such as 2F5/2, 7/2 of the Ce3+ ion. The maximum emission intensity is observed for the asprepared sample (75 °C dried), and the intensity decreases with the increase of sonication time. Figure 10 depicts the photoluminescence excitation (PLE) and photoluminescence (PL) spectra of 1.0 mol % Ce3+-doped

J. Phys. Chem. C, Vol. 114, No. 2, 2010 721 Na(Y1.5Na0.5)F6 nanocrystals heated at different temperatures. Two excitation peaks near 256 and 289 nm are obtained for the as-prepared sample. As the temperature is increased, interlayer water molecules were removed (evidenced by XRD, FTIR, and TGA studies), followed by decreasing the crystal field strength on the activator Ce3+ ion. Thus, the intensity of the 289 nm excitonic peak decreases with increasing the temperature. The peak at 289 nm is not obtained after removing the layer structure (after 300 °C). Similarly, maximum emission intensity is observed for the as-prepared sample (75 °C dried) and the intensity decreases with increase of heating temperature. Matsumoto et al.6 reported similar results in a titanate layered oxide structure intercalated with Eu3+ in TiO2 thin films. They showed higher emission intensity for the Eu3+ ion at room temperature than the 400 °C heated sample. According to them, the interlayer water molecules will be fixed via hydrogen bonding, as in ice, leading to radiation less quenching. This kind of icelike behavior of the water molecules due to strong hydrogen bonding was observed in confined water molecules, as in montmorillonite or other layered or porous compounds.42 Thus, the presence of “icelike connective water” for as-prepared samples is confirmed by FTIR studies (Figure 8), which is further removed on heating or sonication. There are other reasons for decreasing the PL intensity of the Ce3+ ion with increasing the temperature of heating. With increasing the temperature, the equilibrium of the redox system (Ce4+ S Ce3+) changes by direct ionization or by capturing a hole created in the valence states of the anions.43

Ce3+ + h f Ce4+

(5)

Besides this, morphology changes from nanorods to nanoparticles by sonication or heating have significant influence on the luminescence properties of nanostructured materials.44 Song et al.44a reported the enhancement of quantum luminescent efficiency in nanowires compared with nanoparticles because of improvement of crystal anisotropy, which enhances oscillator strength. In the present study, photoluminescence intensity increases with change in shape from nanoparticles to nanorods, which is consistent with Song’s work. Conclusion Our result highlights the effects of ultrasonication and heating temperature on the decomposition of the layered structured fluoride material. Lattice strain changes from compressive to tensile with changing sonication time. Lattice strain is also being influenced by the layered structure. A mechanism has been proposed for changing the morphology from nanorod to hollow nanoparticle under sonication. A marked difference in -OH stretching vibration (3000-3700 cm-1) is obtained after sonication. The -OH stretching vibration bands at 3104 and 3186 cm-1 are shifted to 3432 cm-1, and the band becomes broadened after sonication. Two PLE peaks at 256 and 290 nm of Ce3+ ions confirmed the generation of a strong crystal field due to interlayer water. Acknowledgment. The Department of Science and Technology (NSTI) and “Ramanujan Fellowship” are acknowledged for their financial support. P.G. thanks the Council of Scientific and Industrial Research (CSIR) for providing an S.R.F. (Ext) fellowship. A.K. also thanks the CSIR for providing a fellowship. The authors thank reviewers for their helpful comments.

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